This invention relates to independent sideband (ISB) AM multiphonic systems, such as stereophonic systems for example, and in particular to transmitters for use therein where a second harmonic correction is provided to the stereo difference signal component as phase modulated on the carrier.
U.S. Pat. No. 3,218,393 discloses a system for transmitting stereophonic (stereo) information in an AM broadcast signal. The stereo signal is compatible with existing AM receivers. In accordance with that prior art system, the transmitter of which is shown in the FIG. 1 block diagram, separate L and R stereo signals are combined in subtracting circuit 10 and adding circuit 12, shown in FIG. 1, to form a stereo difference signal L-R and a stereo sum signal L+R. Phase shift networks 14 and 16 are provided to cause the sum and difference signals to have substantially a 90.degree. audio phase difference.
A carrier signal originating at oscillator 18 is first phase modulated (PM) in phase modulator 20 with the phase shifted stereo difference signal information, amplitude limited in limiter 21 and then amplitude modulated in amplitude modulator 22 (AM) with the phase shifted stereo sum signal information. The output signal from amplitude modulator 22 is an independent sideband AM signal wherein the L and R stereo signal information appears separately in the lower and upper sidebands, respectively, of the AM signal.
The carrier may be generated and modulated at the frequency to be transmitted, but it is conventional in such transmitters to modulate a lower frequency carrier and then increase the carrier frequency to the frequency to be transmitted. Consequently, the term "carrier signal" is used herein to refer to both transmission frequency signals and lower frequency signals.
In order to illustrate the mathematics of the prior art AM stereo system transmitter shown in FIG. 1, it is convenient to assume the R signal has zero amplitude, in which case the stereo sum signal (L+R) is equal to the stereo difference signal (L-R). The 90.degree. phase difference between the stereo sum and stereo difference signals, and the use of amplitude and phase modulation, with perpendicular modulation vectors, as illustrated in FIG. 4, results in a composite modulation phasor 24 which precesses around the carrier vector 26 with a single sense of rotation, for example, clockwise. This represents a single-sideband signal.
The ideal situation represented in FIG. 4 presupposes the existence of only a carrier and a fundamental upper sideband. This signal format is not the optimum for a compatible AM stereo system, since the envelope detection characteristics of standard AM receivers will demodulate therefrom a stereo sum signal (L+R) which is a distorted sine wave, shown as 28 in FIG. 5. The ideal detected signal, when the modulation consists of a single tone sine wave on only one of the stereo channels (L or R), is the natural sine wave 30, also shown in FIG. 5.
The actual composite signal generated by the prior art transmitter shown in FIG. 1 includes second harmonic AM and PM components, which are incidentally generated by the limiter-amplitude modulator combination 21-22 due to the multiplicative nature of the amplitude modulation process. The second harmonic AM component renders the signal detected by the envelope detector of a conventional AM receiver relatively distortion free. Since such a monophonic receiver essentially ignores the PM components, they do not affect the compatibility of the transmitted ISB AM stereo signal. However, the second harmonic PM component is almost twice what it should be for a true single sideband signal.
Prior U.S. Pat. No. 3,908,090 describes a transmitter for an ISB AM stereo system wherein a second harmonic correction of the stereo difference signal (L-R) is provided in order to reduce to the desired value of the second harmonic PM component which exists in the ISB AM stereo signal generated by the system. The improved prior art transmitter disclosed in U.S. Pat. No. 3,908,090, and shown in FIG. 2 hereof, includes components which are similar to those in the FIG. 1 system and which bear the same reference numerals. In addition, there is provided a circuit for adding a second harmonic correction to the stereo difference signal (L-R) prior to phase modulation of the carrier. The circuit includes phase shift networks 32 and 34 which provide the separate L and R stereo signals with a phase which is between that of the phase shifted stereo difference signal and the phase shifted stereo sum signal. Constant gain frequency doublers 36 and 38 are provided to double the frequency of the separate L and R stereo signals. The frequency doubled signals are then combined in subtractor 40. Variable gain amplifier 42 is responsive to the amplitude of the phase shifted stereo difference (L-R) signal, as detected in rectifier 44, and supplies a correction signal to adder 46. The correction signal is proportional to the square of the stereo difference signal amplitude and has a frequency of double the audio frequency of the stereo signal. The maximum amplitude of the correction signal is approximately 13% of the maximum amplitude of the stereo difference signal. The modified stereo difference signal that appears at the output of adder 46 is supplied to phase modulator 20 to modulate the carrier, after which the phase modulated carrier is amplitude in modulator 22 by the phaseshifted stereo sum signal (L+R) prior to transmission. Although this second harmonic correction of the stereo difference signal fully corrects for the excessive second harmonic PM component inherently generated in amplitude modulator 22, the desired second harmonic PM component represents distortion in the L-R phase modulation. Since a monophonic receiver essentially ignores the phase modulation in a received ISB AM stereo signal, compatibility of the signal is not affected. Furthermore, this L-R distortion can be cancelled in an ISB stereo receiver to provide a substantially distortion free L-R signal.
While this prior art transmitter provides the desired second harmonic correction for the transmitted ISB AM stereo signal, it should be evident from the drawing of FIG. 2 that considerable transmitter complexity is required for generating the correction signal.
U.S. Pat. No. 4,018,994 discloses a prior art ISB AM stereo receiver arrangement wherein amplitude modulation is used to remove from the received stereo difference signal component (L-R) the second harmonic correction component produced by the ISB AM stereo transmitter disclosed in U.S. Pat. No. 3,908,090 and shown in FIG. 2 hereof. In general, U.S. Pat. No. 4,018,994 discloses that the received stereo difference signal component may be amplitude modulated with one or more signals derived from the stereo sum signal component in order to reduce the L-R distortion which results from the second harmonic correction component produced by such a transmitter.
It is therefore an object of the present invention to provide a new and improved compatible independent sideband multiphonic, for example stereophonic, AM transmitter wherein a desired second harmonic correction of the stereo difference signal can be provided using a simple and economical circuit arrangement.
It is another object of the present invention to provide a new and improved ISB AM multiphonic, for example stereophonic, system wherein the transmitter and the receiver may be proportioned to provide a selected amount of linearity and independence with respect to the transmission of L and R signals through the system, providing a system with low distortion including, particularly, low intermodulation distortion.
It is still another object of the present invention to provide a new and improved ISB AM multiphonic, for example stereophonic, receiver decoder wherein the difference signal component of the received ISB AM stereo signal is modified in an inverse modulator by a selected non linear function of the sum signal component to reduce distortion which is present in the stereo difference signal component of the received ISB signal.
As used herein and in the appended claims, the term "inverse modulation" means the process whereby a first signal (A) is modulated by a second signal (B) in accordance with a selected modulation function having the general form 1/f(B).